This invention relates to the production of hinged paperboard products, and more particularly to a continuous flexible hinge for paperboard products such as commonly used in the making of book cover blanks.
The prior art provides numerous examples of book cover blanks formed from sheets of paperboard material parallel to and spaced from one another by a distance defining a hinge region, and joined in this region by a thin, flexible hinge material. The webs of hinge material and paperboard are continuously fed and laminated, then cut into discrete lengths to form blanks for book covers and the like. Frequently the leafboard portions are formed by laminating several plys of paperboard for additional thickness and strength. U.S. Pat. Nos. 2,922,172, 3,206,226 and 3,206,349 provide examples wherein the hinge material extends entirely across the book cover blanks, from the outer edge of one leafboard to the outer edge of the other. This is commonly referred to as a full width hinge overlay.
U.S. Pat. Nos. 3,145,033 and 3,199,896 relate to continuous hinged paperboard products in which a narrow hinge material, commonly referred to as a hinge tape, is used in the hinge region. Hinge tapes are often preferred since a more expensive hinge material may be used which is better suited to its function, without wasting such material across the entire width of the paperboard. As pointed out in these patents, however, a hinge tape can produce a hump or bump in the middle of the blank. This can interfere with proper stacking of the blanks and may be visible as a line or ridge after a cover is applied to the blank. One solution, which is discussed in these references, is to deform the paperboards in the region of the hinge tape by compressing them beyond their elastic limits, to form a permanent dent in the paperboards of a depth similar to the thickness of the hinge tape. This is done by compressing the paperboards between rollers, one of which has an integral collar which provides the necessary additional compression in the region of the hinge tape.
While the above prior art practices have provided acceptable results and received wide commercial acceptance, there nevertheless remains a need for greater precision, economy, and flexibility. For example, a full width overlay often requires compromises in the choice of hinge material, lest expenses become too great. A narrower compressed hinge tape requires the use of a special, collared roller, having a collar of exactly the width desired. Such rollers are usually sizeable, can indent only one specific width, may be costly to produce, and take time to change. The flexibility of machines using such collared rollers is thus limited, since it can be expensive to change the width and/or depth of the special compression collar at frequent intervals.
A greater objection to compressing or densifying the paperboard in the hinge tape area is the variable thickness or caliper that results from this type of operation. That is, for a given set of circumstances and the particular machine set up, the resulting hinge/paperboard combination thickness can vary throughout the run depending upon the uniformity of the thickness of the paperboard which is supplied, variations in the moisture content of the paperboard, and variations in the amount of "spring back" after being compressed or densified. This can also be affected by variables introduced into the paperboard when it was manufactured.
Such variables in the properties of the paperboard result in a lack of precision in controlling the uniformity of the thickness in the hinge area as the book cover product is being manufactured. Even a slight variation in the thickness of the product in the hinge area, when multiplied many times as the book covers are stacked for shipment, can result in an unattractive load which is difficult to package, and difficult to ship with confidence that it will be delivered to the customer in good, undamaged condition.
A need thus remains for a narrow hinge tape configuration, method, and apparatus for continuous flexible hinges for paperboard and the like wherein the hinge tape will be uniformly and reliably flush with the surface of the paperboards, and will provide for easy and rapid set up, adjustments, and changes in the thickness and/or width of the hinge tape material, all at minimum cost.